With the aim of improving adherence, it is known to incorporate into rubber a formaldehyde acceptor such as a meta-substituted phenol (for example resorcinol or an m-aminophenol) and a formaldehyde donor capable of producing formaldehyde by heating in the rubber.
The abovementioned m-substituted phenols are very volatile. Also, when employed to improve the adherence of rubbers, they release noxious fumes at the vulcanization temperature (generally higher than 110.degree. C.).
In addition, these formaldehyde acceptors are hygroscopic, and this leads to the formation of efflorescence in the rubber mixes during storage. Such efflorescence causes nonuniform adherence between the rubber and the reinforcing material, and this is detrimental to the quality of the vulcanized articles.
To solve these problems it has been proposed to employ a "resorcinol resin" as formaldehyde acceptor, which resin is obtained by condensation of resorcinol with formaldehyde (Industrial Engineering Chemistry, pp. 381-386 (1946)).
Although the resin makes it possible at the same time to obtain a high adherence and an improvement in the physical properties of the rubber (hardness and modulus), it exhibits such a state of deliquescence that it adheres packaging materials or forms a block. It is obvious that this presents many problems during storage or handling. In addition, the resin contains a large quantity of free resorcinol (.gtoreq.15%), and this does not make it possible to solve the disadvantages linked with the presence of smoke and with lump formation.
In EP-A-419 741 and DE-A-4 001 606 it has been proposed to employ modified novolacs resulting from the cocondensation of a phenol (for example resorcinol), of an unsaturated hydrocarbon (for example styrene) and of an aldehyde (for example formaldehyde).
Ternary products of cocondensation of a phenol such as resorcinol, of an alkylphenol and of an aldehyde have also been proposed. For example, in FR 2 193 046, FR 2 223 391, FR 2 392 049 and EP-A-602 861 the said products are obtained by condensation of a resol (alkylphenol/formalin) with resorcinol in the presence of an acidic catalyst.
U.S. Pat. No. 5,030,692 describes the synthesis of a ternary compound by condensation of resorcinol with a resol obtained by reaction of an alkylphenol and of methylformcel or of furfural in acidic medium.
In the ternary products which have just been mentioned, while the free resorcinol content is undoubtedly low (&lt;5%), it is sometimes accompanied by a loss in the reactivity towards the formaldehyde donor.
JP 62004720 describes novolak resins obtained by condensation of resorcinol in exces with dicyclopentadiene (resorcinol/dicyclopentadiene molar ratio=2.9), the exces of resorcinol at the end of the reaction being removed for example by reduced pressure distillation or fractional precipitation. These resins have enhanced heat resistance and flexibility that render them appreciated in the field of electronic materials.
Despite the disclosures of the prior art, there remains a need to obtain resins which do not produce smoke or lump formation and which are found to be as reactive as the resorcinol/formalin resins when rubber is vulcanized.